GENEVA — The World Health Organization told its member nations it was declaring a swine flu pandemic Thursday, the first global flu epidemic in 41 years, as infections climbed in the United States, Europe, Australia, South America and elsewhere.

In a statement sent to health officials, WHO said it decided to raise the pandemic warning level from phase 5 to 6, its highest alert, after holding an emergency meeting with its flu experts.

WHO chief Dr. Margaret Chan was expected to make a formal announcement on the pandemic later Thursday.

The long-awaited pandemic decision is scientific confirmation that a new flu virus has emerged and is quickly circling the globe. It will trigger drugmakers to speed up production of a swine flu vaccine and prompt governments to devote more money toward efforts to contain the virus.

"At this early stage, the pandemic can be characterized globally as being moderate in severity," WHO said in the statement, urging nations not to close borders or restrict travel and trade.

WHO also told countries it was in "close dialogue" with flu vaccine makers and it believed the firms would work "to ensure the largest possible supply of pandemic vaccine in the months to come."

Flu vaccine makers like GlaxoSmithKline PLC and Sanofi-Aventis have been working since last month on a swine flu vaccine. GlaxoSmithKline spokesman Stephen Rea said the company was ready to start making swine flu vaccine in large quantities once it finished its regular flu vaccine production in July.

On Wednesday, WHO said 74 countries had reported nearly 27,737 cases of swine flu, including 141 deaths. The agency has stressed that most cases have been mild and required no treatment, but the fear is that a rash of new infections could overwhelm hospitals and health authorities, especially in poorer countries.

Still, about half of the people who have died from swine flu, also known by its scientific name H1N1, were previously young and healthy - people who are not usually susceptible to flu.

Swine flu is also continuing to spread during the start of summer in the northern hemisphere. Normally, flu viruses disappear with warm weather, but swine flu is proving to be resilient.

The last pandemic, the Hong Kong flu of 1968, killed about 1 million people. Ordinary flu kills about 250,000 to 500,000 people each year.

Many health experts say WHO's pandemic declaration could have come weeks earlier but the agency became bogged down by politics. In May, several countries urged WHO not to declare a pandemic, fearing it would cause social and economic turmoil.

"This is WHO finally catching up with the facts," said Michael Osterholm, a flu expert at the University of Minnesota who has advised the U.S. government on pandemic preparations.

Despite WHO's hopes, raising the epidemic alert to the highest level will almost certainly spark some panic about spread of swine flu.

Fear has already gripped Argentina, where thousands worried about swine flu flooded into hospitals this week, bringing emergency health services in Buenos Aires, the capital, to the brink of collapse. Last month, a bus arriving in Argentina from Chile was stoned by people who thought a passenger on it had swine flu. Chile has the most swine flu cases in South America.

In Hong Kong on Thursday, the government ordered all kindergartens and primary schools closed for two weeks after a dozen students tested positive for swine flu _ a move that some health experts would consider an overreaction.

In the United States, where there have been more than 13,000 cases and at least 27 deaths from swine flu, officials at the U.S. Centers for Disease Control and Prevention said the move would not change how the U.S. tackled swine flu.

"Our actions in the past month have been as if there was a pandemic in this country," Glen Nowak, a CDC spokesman, said Thursday.

The U.S. government has already taken steps like increasing availability of flu-fighting medicines and authorizing $1 billion for the development of a new vaccine against the novel virus. In addition, new cases seem to be declining in many parts of the country, U.S. health officials say, as North America moves out of its traditional winter flu season.

Still, Osterholm said Thursday's decision was a wake-up call for the world.

"I think a lot of people think we're done with swine flu, but you can't fall asleep at the wheel," he said. "We don't know what's going to happen in the next 6 to 12 months."

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